The long-range objectives of Research Project I are the identification, characterization, and validation of etiologically distinct subtypes or dimensions of learning disabilities. To accomplish these objectives, an extensive psychometric test battery will be administered to a sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair is reading disabled, to an independent sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair has ADHD, to a comparison sample of 50 pairs of identical twins and 50 pairs of fraternal twins with no school history of reading deficits or ADHD, and to the siblings of all twin pairs. In collaboration with investigators from Research Projects II-IV, resulting data will be used to assess the genetic and environmental etiologies of reading deficits (including word recognition skills, reading fluency, comprehension and spelling), ADHD, and their comorbidity, as well as their covariation with measures of other psychopathology, reading, language and perceptual processes, and executive functions. These data will also be used to test novel hypotheses of differential etiology of reading difficulties as a function of ADHD dimensions or subtypes, age, gender, and cognitive ability, and to assess the hypothesis that the etiology of deviant scores differs from that of individual differences within the normal range.